The present invention relates to non-magnetic composite particles, a process for producing the non-magnetic composite particles, and a magnetic recording medium using the non-magnetic composite particles. More particularly, the present invention relates to non-magnetic composite particles having a high polishing effect and a high dispersibility, a magnetic recording medium which is provided with a non-magnetic undercoat layer using the above non-magnetic composite particles, and has an excellent durability and a sufficient surface smoothness, a non-magnetic substrate containing the non-magnetic composite particles, and a process for producing the non-magnetic composite particles.
With the recent tendency toward miniaturization and weight-reduction of video or audio magnetic recording and reproducing apparatuses as well as prolonged recording time of these apparatuses, magnetic recording media such as magnetic tapes or magnetic discs have been strongly required to have a high performance, namely, high recording density, high output characteristics such as, especially, improved frequency characteristics, low noise level or the like.
In particular, recent video tapes have been also required to exhibit higher picture quality, so that the frequencies of carrier signals recorded thereon are shifted to shorter wavelength region (short wave-recording) as compared to those used for conventional video tapes. As a result, the magnetization depth from the surface of the magnetic tape has become remarkably shallow.
In order to improve the high output characteristics, especially, the S/N ratio of a magnetic recording medium when short-wavelength signals are recorded thereon, it has been strongly required to reduce the thickness of a magnetic recording layer of the magnetic recording medium. In order to reduce the thickness of the magnetic recording layer, it is necessary to smoothen the surface of the magnetic recording layer and lessen the un-uniformity of thickness thereof. To meet these requirements, it is also required to smoothen the surface of a base film used in the magnetic recording medium.
With the recent tendency toward reduction in thickness of the magnetic recording layer, in order to solve conventional problems such as poor surface properties and deteriorated electromagnetic performance of the magnetic recording layer, there has been proposed and put into practice a method of providing at least one undercoat layer comprising a binder resin and non-magnetic particles dispersed therein (hereinafter referred to merely as xe2x80x9cnon-magnetic undercoat layerxe2x80x9d) on a non-magnetic base film (Japanese Patent Publication (KOKOKU) No. 6-93297(1994), and Japanese Patent Application Laid-Open (KOKAI) Nos. 62-159338(1987), 63-187418(1988), 4-167225(1992), 4-325915(1992), 5-73882(1993) and 5-182177(1993)).
However, these non-magnetic undercoat layers have been strongly required to have further improved surface smoothness. For this reason, it has been attempted to improve the dispersibility of acicular hematite particles used as non-magnetic particles in the non-magnetic undercoat layer.
In addition, the reduced thickness of the magnetic recording layer causes deterioration in durability of the magnetic recording medium itself. Therefore, it has also been strongly required to enhance the durability of the magnetic recording medium.
In order to enhance the durability of the magnetic recording medium itself, abrasives, e.g., oxide particles such as alumina, are added to the magnetic layer or non-magnetic undercoat layer thereof. However, the addition of these abrasives causes many problems. For instance, it is known that alumina has a poor dispersibility in binder resins. Therefore, when a large amount of alumina is added to these layers, there arise defects such as increased dropouts and deteriorated surface smoothness of the obtained magnetic recording medium. In consequence, it has been demanded to provide a non-magnetic undercoat layer and non-magnetic particles used therefor which are capable of imparting a sufficient durability to the obtained magnetic recording medium even when the magnetic recording layer has a small thickness and the amount of abrasives added such as alumina is reduced.
Conventionally, in order to improve various properties of non-magnetic particles, there are known non-magnetic particles having a surface coat composed of an Si compound or an Al compound (Japanese Patent Application Laid-Open (KOKAI) Nos. 5-182177(1993), 5-347017(1993), 6-60362(1994), 10-21532(1998), 10-320753(1993), etc.), or non-magnetic particles on the surface of which fine particles of an Al compound or an Si compound are adhered (Japanese Patent Application Laid-Open (KOKAI) No. 7-192248(1995), etc.).
The non-magnetic particles having a surface coat composed of an Si compound or an Al compound as described in Japanese Patent Application Laid-Open (KOKAI) Nos. 5-182177(1993), 5-347017(1993), 6-60362(1994), 10-21532(1998) and 10-320753(1993) exhibit an improved dispersibility. However, when these non-magnetic particles are used for a non-magnetic undercoat layer, the obtained magnetic recording medium has an insufficient durability. Therefore, the use of these non-magnetic particles cannot reduce the content of abrasives in the magnetic recording medium.
Also, in Japanese Patent Application Laid-Open (KOKAI) No. 7-192248(1995), there is described a method of precipitating fine particles of an oxide or hydroxide of Al or Si on the surface of each non-magnetic particle and then fixing the fine particles thereon by compaction and pulverization treatments. However, as shown in Comparative Examples hereinafter, a considerable amount of the fine particles are desorbed or fallen-off from the surface of each non-magnetic particle. Therefore, when the non-magnetic particles are used for a non-magnetic undercoat layer of a magnetic recording medium, the dispersibility thereof is unsatisfactory, so that the obtained magnetic recording medium cannot show a sufficient durability. Accordingly, the use of such non-magnetic particles cannot reduce the content of abrasives in the magnetic recording medium.
As a result of the present inventors"" earnest studies for solving the above problems, it has been found that by mixing non-magnetic particles having an average particle size of 0.01 to 0.3 xcexcm with inorganic fine particles having an average particle size of 0.001 to 0.07 xcexcm; adhering the inorganic fine particles onto the surface of each non-magnetic particle; adding tetraalkoxysilanes to the obtained particles; and then heating the resultant mixture at a temperature of 40 to 200xc2x0 C. to fix or anchor the inorganic fine particles onto the surface of each magnetic particle through a silicon compound derived from the tetraalkoxysilanes,
the thus obtained non-magnetic composite particles are free from desorption or falling-off of the inorganic fine particles from the surface of each non-magnetic particle, and as a result, can show an excellent dispersibility and a high polishing effect. The present invention has been attained on the basis of this finding.
It is an object of the present invention to provide non-magnetic composite particles for a non-magnetic undercoat layer of a magnetic recording medium, which can exhibit not only an excellent dispersibility but also an excellent polishing effect by firmly fixing or anchoring inorganic fine particles on the surface of each non-magnetic particle.
It is another object of the present invention to provide a non-magnetic substrate for a high-density magnetic recording medium which is excellent in durability and surface smoothness.
It is still another object of the present invention to provide a high-density magnetic recording medium exhibiting excellent durability and surface smoothness.
To accomplish the aims, in a first aspect of the present invention, there are provided non-magnetic composite particles comprising:
(a) non-magnetic particles as core particles having an average particle size of 0.01 to 0.3 xcexcm; and
(b) inorganic fine particles having an average particle size of 0.001 to 0.07 xcexcm, which are present on the surface of each non-magnetic particle, and comprise at least one inorganic compound selected from the group consisting of oxides, nitrides, carbides and sulfides containing aluminum element, zirconium element, cerium element, titanium element, silicon element, boron element or molybdenum element,
the said inorganic fine particles being fixed or anchored on the surface of each non-magnetic particle through a silicon compound derived from tetraalkoxysilanes and the amount of the inorganic fine particles being 0.1 to 20% by weight based on the weight of the non-magnetic particles.
In a second aspect of the present invention, there are provided non-magnetic composite particles comprising:
(a) non-magnetic particles as core particles having an average particle size of 0.01 to 0.3 xcexcum;
(a1) an undercoat formed on the surface of each non-magnetic particle as core particle and comprising at least one compound selected from the group consisting of a hydroxide of aluminum, an oxide of aluminum, a hydroxide of silicon and an oxide of silicon; and
(b) inorganic fine particles having an average particle size of 0.001 to 0.07 xcexcm which are present on the surface of the undercoat, and comprise at least one inorganic compound selected from the group consisting of oxides, nitrides, carbides and sulfides containing aluminum element, zirconium element, cerium element, titanium element, silicon element, boron element or molybdenum element,
the said inorganic fine particles being fixed or anchored on the surface of each non-magnetic particle through a silicon compound derived from tetraalkoxysilanes and the amount of the inorganic fine particles being 0.1 to 20% by weight based on the weight of the non-magnetic particles.
In a third aspect of the present invention, there is provided a non-magnetic substrate for magnetic recording medium, comprising:
(1) a base film; and
(2) a non-magnetic undercoat layer formed on the base film, comprising a binder resin and non-magnetic composite particles comprising:
(a) non-magnetic particles as core particles having an average particle size of 0.01 to 0.3 xcexcm; and
(b) inorganic fine particles having an average particle size of 0.001 to 0.07 xcexcm, which are present on the surface of each non-magnetic particle, and comprise at least one inorganic compound selected from the group consisting of oxides, nitrides, carbides and sulfides containing aluminum element, zirconium element, cerium element, titanium element, silicon element, boron element or molybdenum element,
the said inorganic fine particles being fixed or anchored on the surface of each non-magnetic particle through a silicon compound derived from tetraalkoxysilanes and the amount of the inorganic fine particles being 0.1 to 20% by weight based on the weight of the non-magnetic particles.
In a fourth aspect of the present invention, there is provided a non-magnetic substrate for magnetic recording medium, comprising:
(1) a base film; and
(2) a non-magnetic undercoat layer formed on the base film, comprising a binder resin and non-magnetic composite particles comprising:
(a) non-magnetic particles as core particles having an average particle size of 0.01 to 0.3 xcexcm;
(a1) an undercoat formed on the surface of each non-magnetic particle as core particle and comprising at least one compound selected from the group consisting of a hydroxide of aluminum, an oxide of aluminum, a hydroxide of silicon and an oxide of silicon; and
(b) inorganic fine particles having an average particle size of 0.001 to 0.07 xcexcm, which are present on the surface of the undercoat, and comprise at least one inorganic compound selected from the group consisting of oxides, nitrides, carbides and sulfides containing aluminum element, zirconium element, cerium element, titanium element, silicon element, boron element or molybdenum element,
the said inorganic fine particles being fixed or anchored on the surface of each non-magnetic particle through a silicon compound derived from tetraalkoxysilanes and the amount of the inorganic fine particles being 0.1 to 20% by weight based on the weight of the non-magnetic particles.
In a fifth aspect of the present invention, there is provided a magnetic recording medium comprising:
(1) a non-magnetic base film;
(2) a non-magnetic undercoat layer formed on the base film, comprising a binder resin and non-magnetic composite particles; and
(3) a magnetic recording layer formed on the non-magnetic undercoat layer, comprising magnetic particles and a binder resin,
the said non-magnetic composite particles comprising:
(a) non-magnetic particles as core particles having an average particle size of 0.01 to 0.3 xcexcm; and
(b) inorganic fine particles having an average particle size of 0.001 to 0.07 xcexcm, which are present on the surface of each non-magnetic particle, and comprise at least one inorganic compound selected from the group consisting of oxides, nitrides, carbides and sulfides containing aluminum element, zirconium element, cerium element, titanium element, silicon element, boron element or molybdenum element,
the said inorganic fine particles being fixed or anchored on the surface of each non-magnetic particle through a silicon compound derived from tetraalkoxysilanes and the amount of the inorganic fine particles being 0.1 to 20% by weight based on the weight of the non-magnetic particles.
In a sixth aspect of the present invention, there is provided a magnetic recording medium comprising:
(1) a non-magnetic base film;
(2) a non-magnetic undercoat layer formed on the base film, comprising a binder resin and non-magnetic composite particles; and
(3) a magnetic recording layer formed on the non-magnetic undercoat layer, comprising magnetic particles and a binder resin,
the said non-magnetic composite particles comprising:
(a) non-magnetic particles as core particles having an average particle size of 0.01 to 0.3 xcexcm;
(a1) an undercoat formed on the surface of each non-magnetic particle as core particle and comprising at least one compound selected from the group consisting of a hydroxide of aluminum, an oxide of aluminum, a hydroxide of silicon and an oxide of silicon; and
(b) inorganic fine particles having an average particle size of 0.001 to 0.07 xcexcm, which are present on the surface of the undercoat, and comprise at least one inorganic compound selected from the group consisting of oxides, nitrides, carbides and sulfides containing aluminum element, zirconium element, cerium element, titanium element, silicon element, boron element or molybdenum element,
the said inorganic fine particles being fixed or anchored on the surface of each non-magnetic particle through a silicon compound derived from tetraalkoxysilanes and the amount of the inorganic fine particles being 0.1 to 20% by weight based on the weight of the non-magnetic particles.
In a seventh aspect of the present invention, there is provided a process for producing the non-magnetic composite particles as defined in the first aspect, comprising:
(i) mixing non-magnetic particles having an average particle size of 0.01 to 0.3 xcexcm with inorganic fine particles having an average particle size of 0.001 to 0.07 xcexcm, and comprising at least one inorganic compound selected from the group consisting of oxides, nitrides, carbides and sulfides containing aluminum element, zirconium element, cerium element, titanium element, silicon element, boron element or molybdenum element, to adhere the inorganic fine particles onto the surface of each non-magnetic particle;
(ii) adding tetraalkoxysilanes to the resultant particles; and
(iii) heating the obtained mixture at a temperature of 40 to 200xc2x0 C., thereby fixing or anchoring the inorganic fine particles onto the surface of each non-magnetic particle through a silicon compound derived from the tetraalkoxysilanes.
The present invention will be described in detail below.
First, the non-magnetic composite particles according to the present invention are explained.
The non-magnetic composite particles of the present invention, comprise magnetic particles as core particles having an average particle size of 0.01 to 0.3 xcexcm; and inorganic fine particles adhered onto the surface of each magnetic particle, which have an average particle size of 0.001 to 0.07 xcexcm, and comprise at least one inorganic compound selected from the group consisting of oxides, nitrides, carbides and sulfides containing aluminum element, zirconium element, cerium element, titanium element, silicon element, boron element or molybdenum element. The inorganic fine particles are firmly fixed or anchored onto the surface of each magnetic particle through a silicon compound derived from tetraalkoxysilane.
As the non-magnetic particles, acicular hematite particles, acicular iron oxide hydroxide particles and the like may be exemplified.
The non-magnetic particles as core particles used in the present invention may have either an acicular shape. The xe2x80x9cacicularxe2x80x9d non-magnetic particles include xe2x80x9cspindle-shapedxe2x80x9d particles, xe2x80x9crice grain-shapedxe2x80x9d particles and the like in addition to literally xe2x80x9cneedle-likexe2x80x9d particles.
The non-magnetic particles as core particles have an average major axial diameter of usually 0.01 to 0.3 xcexcm, preferably 0.02 to 0.2 xcexcm.
When the average major axial diameter of the non-magnetic particles as core particles is more than 0.3 xcexcm, the obtained non-magnetic composite particles may become coarse. When such coarse particles are used to form a non-magnetic undercoat layer of magnetic recording medium, the obtained non-magnetic undercoat layer may be deteriorated in surface smoothness. When the average major axial diameter of the non-magnetic particles is less than 0.01 xcexcm, the non-magnetic particles may become extremely fine, so that the agglomeration of the non-magnetic particles tends to occur due to the increased intermolecular force therebetween. As a result, it is difficult to uniformly adhere the inorganic fine particles onto the surface of each non-magnetic particle and evenly fix or anchor the inorganic fine particles thereonto through the silicon compound derived (produced) from tetraalkoxysilane.
The ratio of an average major axial diameter to an average minor axial diameter (hereinafter referred to merely as xe2x80x9caspect ratioxe2x80x9d) of the acicular non-magnetic particles is usually 2:1 to 15:1, preferably 3:1 to 10:1.
When the aspect ratio of the acicular non-magnetic particles is more than 15:1, the non-magnetic particles may tend to be entangled or intertwined with each other. As a result, it may be difficult to uniformly adhere the inorganic fine particles onto the surface of each non-magnetic particle and evenly fix or anchor the inorganic fine particles thereonto through the silicon compound derived from tetraalkoxysilane. When the aspect ratio of the acicular non-magnetic particles is less than 2:1, the obtained non-magnetic undercoat layer may be deteriorated in strength.
The non-magnetic particles as core particles used in the present invention have preferably a geometrical standard deviation of major axial diameter of usually not more than 2.0, more preferably not more than 1.8, still more preferably not more than 1.6. When the geometrical standard deviation of particle size of the non-magnetic particles is more than 2.0, coarse particles may exist in the non-magnetic particles, thereby inhibiting the non-magnetic particles from being uniformly dispersed. As a result, it may be difficult to uniformly adhere the inorganic fine particles onto the surface of each non-magnetic particle and evenly fix or anchor the inorganic fine particles thereonto through the silicon compound derived from tetraalkoxysilane. The lower limit of the geometrical standard deviation is usually 1.01. It is difficult to industrially produce non-magnetic particles having a geometrical standard deviation of particle size of less than 1.01.
The non-magnetic particles as core particles used in the present invention have a BET specific surface area of usually 15 to 150 m2/g, preferably 20 to 120 m2/g, more preferably 25 to 100 m2/g. When the BET specific surface area value of the non-magnetic particles is less than 15 m2/g, the non-magnetic particles may become too coarse or the sintering therebetween tends to be caused, resulting in the production of coarse non-magnetic composite particles. When such coarse non-magnetic composite particles are used to form a non-magnetic undercoat layer, the obtained coating layer may be deteriorated in surface smoothness. When the BET specific surface area value of the non-magnetic particles is more than 150 m2/g, the non-magnetic particles may become extremely fine, so that the agglomeration of the particles tends to occur due to the increased intermolecular force therebetween. As a result, it is difficult to uniformly adhere the inorganic fine particles onto the surface of each non-magnetic particle and evenly fix or anchor the inorganic fine particles thereon through the silicon compound derived from tetraalkoxysilane.
The non-magnetic particles as core particles used in the present invention have a volume resistivity value of usually not more than 5.0xc3x97109 xcexa9xc2x7cm.
The shape and size of the non-magnetic composite particles according to the present invention varies depending upon those of the non-magnetic particles as core particles, and are analogous thereto.
The non-magnetic composite particles according to the present invention have an average major axial diameter of usually 0.01 to 0.3 xcexcm, preferably 0.02 to 0.2 xcexcm.
When the average major axial diameter of the non-magnetic composite particles according to the present invention is more than 0.3 xcexcm, the particle size become large, so that the non-magnetic undercoat layer formed by using the non-magnetic composite particles tends to have a deteriorated surface smoothness. When the average major axial diameter of the non-magnetic composite particles is less than 0.01 xcexcm, the particles become extremely fine and tend to be agglomerated together due to the increased intermolecular force therebetween, resulting in poor dispersibility in vehicle upon the production of a non-magnetic coating composition.
The non-magnetic composite particles according to the present invention have an aspect ratio of usually 2:1 to 15:1, preferably 3:1 to 10:1.
When the aspect ratio of the non-magnetic composite particles is more than 15:1, the particles may tend to be entangled and intertwined with each other, sometimes resulting in poor dispersibility in vehicle upon the production of a non-magnetic coating composition and increased viscosity of the obtained non-magnetic coating composition. When the aspect ratio of the non-magnetic composite particles is less than 2:1, the non-magnetic undercoat layer of the magnetic recording medium may be deteriorated in strength.
The geometrical standard deviation of particle size of the non-magnetic composite particles according to the present invention is usually not more than 2.0. When the geometrical standard deviation is more than 2.0, a large amount of coarse particles may be present in the non-magnetic composite particles, thereby adversely affecting the surface smoothness of the non-magnetic undercoat layer formed on the non-magnetic base film. In the consideration of the surface smoothness of the obtained non-magnetic undercoat layer, the geometrical standard deviation of particle size of the non-magnetic composite particles is preferably not more than 1.8, more preferably not more than 1.6. Further, in the consideration of industrial productivity, the lower limit of the geometrical standard deviation is usually 1.01, because it is industrially difficult to produce non-magnetic composite particles having a geometrical standard deviation of particle size of less than 1.01.
The non-magnetic composite particles according to the present invention have a BET specific surface area of usually 16 to 160 m2/g, preferably 21 to 130 m2/g, more preferably 26 to 110 m2/g. When the BET specific surface area of the non-magnetic composite particles is less than 16 m2/g, the non-magnetic composite particles may become coarse or the sintering therebetween tends to be caused. The use of such coarse or sintered non-magnetic composite particles leads to deterioration in surface smoothness of the obtained non-magnetic undercoat layer. When the BET specific surface area of the non-magnetic composite particles is more than 160 m2/g, the non-magnetic composite particles may become extremely fine and tend to be agglomerated together due to the increased intermolecular force therebetween, resulting in poor dispersibility in vehicle upon the production of a non-magnetic coating composition.
The non-magnetic composite particles according to the present invention have a volume resistivity value of usually not more than 1.0xc3x971010 xcexa9xc2x7cm, preferably 1.0xc3x97105 to 9.0xc3x97109 xcexa9xc2x7cm, more preferably 1.0xc3x97105 to 8.0xc3x97109 xcexa9xc2x7cm. When the volume resistivity value is more than 1.0xc3x971010 xcexa9xc2x7cm, it may be difficult to lower a surface resistivity value of the magnetic recording medium obtained therefrom.
The percentage of the inorganic fine particles desorbed or fallen-off from the non-magnetic composite particles (desorption percentage) is usually not more than 15% by weight, preferably not more than 12% by weight, more preferably not more than 10% by weight based on the weight of the inorganic fine particles, when measured by the method as defined in Examples below. When the desorption percentage is more than 15% by weight, the inorganic fine particles desorbed may tend to inhibit the non-magnetic composite particles from being uniformly dispersed in vehicle, and the obtained magnetic recording medium may fail to show a sufficient durability and magnetic head cleaning property.
The inorganic fine particles of the non-magnetic composite particles are at least one kind of fine particles composed of at least one inorganic compound selected from the group consisting of oxides, nitrides, carbides and sulfides of aluminum, zirconium, cerium, titanium, silicon, boron or molybdenum.
As the inorganic fine particles used in the present invention, there may be exemplified (a) oxide fine particles such as aluminum oxide fine particles, zirconium oxide fine particles, cerium oxide fine particles, titanium oxide fine particles, silicon oxide fine particles, molybdenum oxide fine particles or the like; (b) nitride fine particles such as aluminum nitride fine particles, titanium nitride fine particles, silicon nitride fine particles, zirconium nitride fine particles, molybdenum nitride fine particles, boron nitride fine particles or the like; (c) carbide fine particles such as aluminum carbide fine particles, silicon carbide fine particles, zirconium carbide fine particles, cerium carbide fine particles, titanium carbide fine particles, boron carbide fine particles, molybdenum carbide fine particles or the like; and (d) sulfide fine particles such as aluminum sulfide fine particles, zirconium sulfide fine particles, titanium sulfide fine particles, silicon sulfide fine particles, molybdenum disulfide fine particles or the like.
In the consideration of the durability of the obtained magnetic recording medium, it is preferred to use at least one fine particles selected from aluminum oxide fine particles, zirconium oxide fine particles, cerium oxide fine particles, aluminum nitride fine particles, titanium nitride fine particles, silicon nitride fine particles, zirconium nitride fine particles, boron nitride fine particles, silicon carbide fine particles, zirconium carbide fine particles, titanium carbide fine particles, boron carbide fine particles, molybdenum carbide fine particles.
The inorganic fine particles used in the present invention have an average particle size of usually 0.001 to 0.07 xcexcm, preferably 0.002 to 0.05 xcexcm.
When the average particle size of the inorganic fine particles is less than 0.001 xcexcm, the particles become extremely fine, resulting in poor handling thereof. When the average particle size of the inorganic fine particles is more than 0.07 xcexcm, the particle size of the inorganic fine particles is too large as compared to that of the non-magnetic particles as core particles, so that the adhesion of the inorganic fine particles onto the non-magnetic particles becomes insufficient.
The amount of the inorganic fine particles adhered onto the non-magnetic particles is usually 0.1 to 20% by weight based on the weight of the non-magnetic particles as core particles.
When the amount of the inorganic fine particles adhered is less than 0.1% by weight, it is difficult to obtain non-magnetic composite particles showing a sufficient polishing effect, due to the too small amount of the inorganic fine particles adhered. On the contrary, when the amount of the inorganic fine particles adhered is more than 20% by weight, the obtained non-magnetic composite particles show a sufficient polishing effect. However, since the amount of the inorganic fine particles adhered is too large, the inorganic fine particles tend to be desorbed or fallen-off from the surface of each non-magnetic particle, thereby failing to obtain non-magnetic undercoat layer having an excellent durability. The amount of the inorganic fine particles adhered onto the non-magnetic particles is preferably 0.15 to 15% by weight, more preferably 0.2 to 10% by weight based on the weight of the non-magnetic particles as core particles.
The silicon compound through which the inorganic fine particles are fixed or anchored onto the surface of each non-magnetic particle is produced by heat-treating tetraalkoxysilanes represented by the following general formula:
SiX4
wherein X represents xe2x80x94OR wherein R is C1-C5 alkyl group.
Examples of the tetraalkoxysilanes may include tetramethoxysilane, tetraethoxysilane, tetrapropoxysilane, tetrabutoxysilane, tetrapentyloxysilane or the like. Among these tetraalkoxysilanes, in the consideration of the anchoring effect of the inorganic fine particles, tetramethoxysilane and tetraethoxysilane are preferred.
The coating amount of the silicon compound produced from tetraalkoxysilane is usually 0.01 to 5.0% by weight, preferably 0.02 to 4.0% by weight, more preferably 0.03 to 3.0% by weight (calculated as Si) based on the weight of the non-magnetic composite particles.
When the coating amount of the silicon compound is less than 0.01% by weight, the inorganic fine particles may not be sufficiently fixed or anchored onto the surface of each non-magnetic particle through the silicon compound derived therefrom and, therefore, tend to be desorbed therefrom, thereby failing to obtain magnetic recording media having an excellent durability and magnetic head cleaning property.
When the coating amount of the silicon compound derived from the tetraalkoxysilane is 0.01 to 5.0% by weight, the inorganic fine particles can be sufficiently fixed or anchored onto the surface of each non-magnetic particle. Therefore, it is meaningless to use the silicon compound in an amount of more than 5.0% by weight.
In the non-magnetic composite particles according to the present invention, the non-magnetic particles as core particles may be preliminarily coated with an undercoating material composed of at least one compound selected from the group consisting of a hydroxide of aluminum, an oxide of aluminum, a hydroxide of silicon and an oxide of silicon (hereinafter referred to as xe2x80x9chydroxides and/or oxides of aluminum and/or siliconxe2x80x9d). The formation of such an undercoat is more advantageous to enhance the dispersibility of the non-magnetic composite particles in vehicle as compared to those having no undercoat.
The amount of the undercoat is preferably 0.01 to 20% by weight (calculated as Al, SiO2 or a sum of Al and SiO2) based on the weight of the non-magnetic particles coated with the undercoat.
When the covering amount of the undercoat is less than 0.01% by weight, it may be difficult to obtain the effect of improving the desorption percentage of inorganic fine particles. When the covering amount of the undercoat is 0.01 to 20% by weight, a sufficient effect of improving the desorption percentage of inorganic fine particles can be obtained. Therefore, it is meaningless to use each non-magnetic particle with the undercoat in an amount of more than 20% by weight.
The non-magnetic composite particles having the undercoat may have the substantially same particle size, geometrical standard deviation value, volume resistivity value and BET specific surface area value as those having no undercoat. By covering each non-magnetic particle with the undercoat, the desorption percentage of the inorganic fine particles can be effectively reduced to preferably not more than 12%, more preferably not more than 10%.
Next, the magnetic recording medium according to the present invention will be described.
The magnetic recording medium according to the present invention comprises:
a non-magnetic substrate comprising a non-magnetic base film, and a non-magnetic undercoat layer formed on the non-magnetic base film, comprising a binder resin and the non-magnetic composite particles; and
a magnetic coating film formed on the non-magnetic substrate, comprising a binder resin and magnetic particles.
As the non-magnetic base film, there may be used those ordinarily used for magnetic recording media. Examples of the non-magnetic base film may include films of synthetic resins such as polyethylene terephthalate, polyethylene, polypropylene, polycarbonates, polyethylene naphthalate, polyamides, polyamideimides and polyimides; foils or plates of metals such as aluminum and stainless steel; or various kinds of papers. The thickness of the non-magnetic base film varies depending upon materials used, and is usually 1.0 to 300 xcexcm, preferably 2.0 to 200 xcexcm.
As the non-magnetic base film for magnetic discs, there may be generally used a polyethylene terephthalate film having a thickness of usually 50 to 300 xcexcm, preferably 60 to 200 xcexcm. As the non-magnetic base film for magnetic tapes, there may be used a polyethylene terephthalate film having a thickness of usually 3 to 100 xcexcm, preferably 4 to 20 xcexcm, a polyethylene naphthalate film having a thickness of usually 3 to 50 xcexcm, preferably 4 to 20 xcexcm, or a polyamide film having a thickness of usually 2 to 10 xcexcm, preferably 3 to 7 xcexcm.
As the binder resins for the non-magnetic undercoat layer, there may also be used those presently ordinarily used for the production of magnetic recording media. Examples of the binder resins may include vinyl chloride-vinyl acetate copolymer resins, urethane resins, vinyl chloride-vinyl acetate-maleic acid copolymer resins, urethane elastomers, butadiene-acrylonitrile copolymer resins, polyvinyl butyral, cellulose derivatives such as nitrocellulose, polyester resins, synthetic rubber-based resins such as polybutadiene, epoxy resins, polyamide resins, polyisocyanates, electron beam-curable acrylic urethane resins, or mixtures thereof.
The respective binder resins may contain a functional group such as xe2x80x94OH, xe2x80x94COOH, xe2x80x94SO3M, xe2x80x94OPO2M2 and xe2x80x94NH2 wherein M represents H, Na or K. In the consideration of the dispersibility of the non-magnetic composite particles in vehicle upon the production of a non-magnetic coating composition, the use of such binder resins containing xe2x80x94COOH or xe2x80x94SO3M as a functional group is preferred.
The amount of the non-magnetic composite particles in the non-magnetic undercoat layer is usually 5 to 2,000 parts by weight, preferably 100 to 1,000 parts by weight based on 100 parts by weight of the binder resin.
When the amount of the non-magnetic composite particles is less than 5 parts by weight, the non-magnetic composite particles may not be continuously dispersed in a coating layer due to the too small content in a non-magnetic coating composition, resulting in insufficient surface smoothness and strength of the obtained coating layer. When the amount of the non-magnetic composite particles is more than 2,000 parts by weight, the non-magnetic composite particles may not be uniformly dispersed in the non-magnetic coating composition due to the too large content as compared to that of the binder resin. As a result, when such a non-magnetic coating composition is coated onto the non-magnetic base film, it is difficult to obtain a coating film having a sufficient surface smoothness. Further, since the non-magnetic composite particles cannot be sufficiently bonded together by the binder resin, the obtained coating film becomes brittle.
The thickness of the non-magnetic undercoat layer formed on the non-magnetic base film is usually 0.2 to 10 xcexcm. When the thickness of the non-magnetic undercoat layer is less than 0.2 xcexcm, it may become difficult to improve the surface roughness of the non-magnetic base film, and the non-magnetic undercoat layer may tend to have a deteriorated strength. In the consideration of strength of the undercoat layer and reduction in thickness of the resultant magnetic recording medium, the thickness of the non-magnetic undercoat layer is preferably 0.5 to 5 xcexcm.
The non-magnetic recording layer may further contain various additives used in ordinary magnetic recording media such as lubricants, abrasives and anti-static agents.
In case of using the non-magnetic composite particles, in which no hydroxides and/or oxides of aluminum and/or silicon coat is formed on the surface of the non-magnetic particles as core particles, the non-magnetic substrate according to the present invention has a gloss (of the coating film) of usually 176 to 300%, preferably 180 to 300%, more preferably 184 to 300%; a surface roughness Ra (of the coating film) of usually 0.5 to 8.5 nm, preferably 0.5 to 8.0 nm; a Young""s modulus (relative value to a commercially available video tape: and AV T-120 produced by Victor Company of Japan, Limited) of usually 124 to 160, preferably 126 to 160; and a surface resistivity of usually 1.0xc3x97105 to 1.0xc3x971013 xcexa9/cm2, preferably 1.0xc3x97105 to 7.5xc3x971012 xcexa9/cm2, more preferably 1.0xc3x97105 to 5.0xc3x971012 xcexa9/cm2.
In case of using the non-magnetic composite particles, in which the hydroxides and/or oxides of aluminum and/or silicon coat is formed on the surface of the non-magnetic particles as core particles, the non-magnetic substrate according to the present invention has a gloss (of the coating film) of usually 180 to 300%, preferably 184 to 300%, more preferably 188 to 300%; a surface roughness Ra (of the coating film) of usually 0.5 to 8.0 nm, preferably 0.5 to 7.5 nm; a Young""s modulus (relative value to a commercially available video tape: and AV T-120 produced by Victor Company of Japan, Limited) of usually 126 to 160, preferably 128 to 160; and a surface resistivity of usually 1.0xc3x97105 to 1.0xc3x971013 xcexa9/cm2, preferably 1.0xc3x97105 to 7.5xc3x971012 xcexa9/cm2, more preferably 1.0xc3x97105 to 5.0xc3x971012 xcexa9/cm2.
As the magnetic particles, there may be used Co-coated magnetic iron oxide particles obtained by coating Co or Co and Fe on magnetic iron oxide particles such as maghemite particles (xcex3-Fe2O3) or magnetite particles (FeOx.Fe2O3: 0 less than xxe2x89xa61); Co-coated magnetic iron oxide particles obtained by incorporating at least one selected from elements other than Fe such as Co, Al, Ni, P, Zn, Si, B and rare earth metals in the above Co-coated magnetic iron oxide particles; acicular magnetic metal particles containing iron as a main component; acicular magnetic iron alloy particles containing at least one selected from elements other than Fe such as Co, Al, Ni, P, Zn, Si, B and rare earth metals; plate-like magnetoplumbite-type ferrite particles containing Ba, Sr or Baxe2x80x94Sr; or plate-like magnetoplumbite-type ferrite particles obtained by incorporating at least one coercive force-reducing agent selected from the group consisting of divalent and tetravalent metals such as Co, Ni, Zn, Mn, Mg, Ti, Sn, Zr, Nb, Cu and Mo in the above plate-like magnetoplumbite-type ferrite particles.
In the consideration of the recent tendency toward high-density recording on magnetic recording media, as the magnetic particles, there may be suitably used the acicular magnetic metal particles containing iron as a main component and the acicular magnetic iron alloy particles containing at least one selected from elements other than Fe such as Co, Al, Ni, P, Zn, Si, B and rare earth metals.
More specifically, the magnetic acicular metal particles containing iron as a main component and acicular magnetic iron alloy particles containing elements other than Fe such as Co, Al, Ni, P, Zn, Si, B and rare earth metals may be exemplified as follows.
1) Magnetic acicular metal particles comprises iron; and cobalt of usually 0.05 to 40% by weight, preferably 1.0 to 35% by weight, more preferably 3 to 30% by weight (calculated as Co) based on the weight of the magnetic acicular metal particles.
2) Magnetic acicular metal particles comprises iron; and aluminum of usually 0.05 to 10% by weight, preferably 0.1 to 7% by weight (calculated as Al) based on the weight of the magnetic acicular metal particles.
3) Magnetic acicular metal particles comprises iron; cobalt of usually 0.05 to 40% by weight, preferably 1.0 to 35% by weight, more preferably 3 to 30% by weight (calculated as Co) based on the weight of the magnetic acicular metal particles; and aluminum of usually 0.05 to 10% by weight, preferably 0.1 to 7% by weight (calculated as Al) based on the weight of the magnetic acicular metal particles.
4) Magnetic acicular metal particles comprises iron; cobalt of usually 0.05 to 40% by weight, preferably 1.0 to 35% by weight, more preferably 3 to 30% by weight (calculated as Co) based on the weight of the magnetic acicular metal particles; and at least one selected from the group consisting of Nd, La and Y of usually 0.05 to 10% by weight, preferably 0.1 to 7% by weight (calculated as the corresponding element) based on the weight of the magnetic acicular metal particles.
5) Magnetic acicular metal particles comprises iron; aluminum of usually 0.05 to 10% by weight, preferably 0.1 to 7% by weight (calculated as Al) based on the weight of the magnetic acicular metal particles; and at least one selected from the group consisting of Nd, La and Y of usually 0.05 to 10% by weight, preferably 0.1 to 7% by weight (calculated as the corresponding element) based on the weight of the magnetic acicular metal particles.
6) Magnetic acicular metal particles comprises iron; cobalt of usually 0.05 to 40% by weight, preferably 1.0 to 35% by weight, more preferably 3 to 30% by weight (calculated as Co) based on the weight of the magnetic acicular metal particles; aluminum of usually 0.05 to 10% by weight, preferably 0.1 to 7% by weight (calculated as Al) based on the weight of the magnetic acicular metal particles; and at least one selected from the group consisting of Nd, La and Y of usually 0.05 to 10% by weight, preferably 0.1 to 7% by weight (calculated as the corresponding element) based on the weight of the magnetic acicular metal particles.
7) Magnetic acicular metal particles comprises iron; cobalt of usually 0.05 to 40% by weight, preferably 1.0 to 35% by weight, more preferably 3 to 30% by weight (calculated as Co) based on the weight of the magnetic acicular metal particles; and at least one selected from the group consisting of Ni, P, Si, Zn, Ti, Cu and B of usually 0.05 to 10% by weight, preferably 0.1 to 7% by weight (calculated as the corresponding element) based on the weight of the magnetic acicular metal particles.
8) Magnetic acicular metal particles comprises iron; aluminum of usually 0.05 to 10% by weight, preferably 0.1 to 7% by weight (calculated as Al) based on the weight of the magnetic acicular metal particles; and at least one selected from the group consisting of Ni, P, Si, Zn, Ti, Cu and B of usually 0.05 to 10% by weight, preferably 0.1 to 7% by weight (calculated as the corresponding element) based on the weight of the magnetic acicular metal particles.
9) Magnetic acicular metal particles comprises iron; cobalt of usually 0.05 to 40% by weight, preferably 1.0 to 35% by weight, more preferably 3 to 30% by weight (calculated as Co) based on the weight of the magnetic acicular metal particles; aluminum of usually 0.05 to 10% by weight, preferably 0.1 to 7% by weight (calculated as Al) based on the weight of the magnetic acicular metal particles; and at least one selected from the group consisting of Ni, P, Si, Zn, Ti, Cu and B of usually 0.05 to 10% by weight, preferably 0.1 to 7% by weight (calculated as the corresponding element) based on the weight of the magnetic acicular metal particles.
10) Magnetic acicular metal particles comprises iron; cobalt of usually 0.05 to 40% by weight, preferably 1.0 to 35% by weight, more preferably 3 to 30% by weight (calculated as Co) based on the weight of the magnetic acicular metal particles; at least one selected from the group consisting of Nd, La and Y of usually 0.05 to 10% by weight, preferably 0.1 to 7% by weight (calculated as the corresponding element) based on the weight of the magnetic acicular metal particles; and at least one selected from the group consisting of Ni, P, Si, Zn, Ti, Cu and B of usually 0.05 to 10% by weight, preferably 0.1 to 7% by weight (calculated as the corresponding element) based on the weight of the magnetic acicular metal particles.
11) Magnetic acicular metal particles comprises iron; aluminum of usually 0.05 to 10% by weight, preferably 0.1 to 7% by weight (calculated as Al) based on the weight of the magnetic acicular metal particles; at least one selected from the group consisting of Nd, La and Y of ordinarily 0.05 to 10% by weight, preferably 0.1 to 7% by weight (calculated as the corresponding element) based on the weight of the magnetic acicular metal particles; and at least one selected from the group consisting of Ni, P, Si, Zn, Ti, Cu and B of usually 0.05 to 10% by weight, preferably 0.1 to 7% by weight (calculated as the corresponding element) based on the weight of the magnetic acicular metal particles.
12) Magnetic acicular metal particles comprises iron; cobalt of usually 0.05 to 40% by weight, preferably 1.0 to 35% by weight, more preferably 3 to 30% by weight (calculated as Co) based on the weight of the magnetic acicular metal particles; aluminum of usually 0.05 to 10% by weight, preferably 0.1 to 7% by weight (calculated as Al) based on the weight of the magnetic acicular metal particles; at least one selected from the group consisting of Nd, La and Y of usually 0.05 to 10% by weight, preferably 0.1 to 7% by weight (calculated as the corresponding element) based on the weight of the magnetic acicular metal particles; and at least one selected from the group consisting of Ni, P, Si, Zn, Ti, Cu and B of usually 0.05 to 10% by weight, preferably 0.1 to 7% by weight (calculated as the corresponding element) based on the weight of the magnetic acicular metal particles.
The iron content in the particles is the balance, and is preferably 50 to 99% by weight, more preferably 60 to 95% by weight (calculated as Fe) based on the weight of the magnetic acicular metal particles containing iron as a main component.
The magnetic particles used in the present invention have an average major axis diameter of usually 0.01 to 0.50 xcexcm, preferably 0.03 to 0.30 xcexcm, an average minor axis diameter of usually 0.0007 to 0.17 xcexcm, preferably 0.003 to 0.10 xcexcm. It is preferred that the shape of the magnetic particles is acicular or plate-like. The acicular shape may include not only needle-shape but also spindle-shape, rice ball-shape, or the like.
In the case that the shape of the magnetic particles is acicular, the magnetic particles have an aspect ratio of usually not less than 3:1, preferably and not less than 5:1. The upper limit of the aspect ratio is usually 15:1, preferably 10:1 with the consideration of the dispersibility in the vehicle.
In the case that the shape of the magnetic particles is plate-like, the magnetic particles have a plate ratio (an average particle size/average plate thickness) of usually not less than 2:1, preferably and not less than 3:1. The upper limit of the plate ratio is usually 20:1, preferably 15:1 with the consideration of the dispersibility in the vehicle.
As to the magnetic properties of the magnetic particles used in the present invention, the coercive force is usually 500 to 4000 Oe (39.8 to 318.3 kA/m), preferably 550 to 4000 Oe (43.8 to 318.3 kA/m), and the saturation magnetization is usually 50 to 170 emu/g (50 to 170 Am2/kg), preferably 60 to 170 emu/g (60 to 170 Am2/kg).
With the consideration of the high-density recording of the magnetic recording medium, as to the magnetic properties of the magnetic metal particles containing iron as a main component and the acicular magnetic iron alloy particles used as magnetic particles, the coercive force is usually 800 to 3500 Oe (63.7 to 278.5 kA/m), preferably 900 to 3500 Oe (71.6 to 278.5 kA/m), and the saturation magnetization is usually 90 to 170 emu/g (90 to 170 Am2/kg), preferably 100 to 170 emu/g (100 to 170 Am2/kg).
As the binder resin for the magnetic recording layer, the same binder resin as that used for the production of the non-magnetic undercoat layer is usable.
The thickness of the magnetic recording layer formed on the surface of the non-magnetic undercoat layer is usually in the range of 0.01 to 5.0 xcexcm. When the thickness is less than 0.01 xcexcm, uniform coating may be difficult, so that unfavorable phenomenon such as un-uniformity on the coating surface is observed. On the other hand, when the thickness exceeds 5.0 xcexcm, it may be difficult to obtain desired electromagnetic performance due to an influence of diamagnetism. The preferable thickness is in the range of 0.05 to 1.0 xcexcm.
The mixing ratio of the magnetic particles with the binder resin in the magnetic recording layer is usually 200 to 2000 parts by weight, preferably 300 to 1500 parts by weight based on 100 parts by weight of the binder resin.
It is possible to add a lubricant, a polishing agent, an antistatic agent, etc. which are generally used for the production of a magnetic recording medium to the magnetic recording layer in an amount of 0.1 to 50 parts by weight based on 100 parts by weight of the binder resin.
The magnetic recording medium of the present invention has a coercive force value of usually 500 to 4,000 Oe (39.8 to 318.3 kA/m), a squareness (residual magnetic flux density Br/saturation magnetic flux density Bm) of usually 0.85 to 0.95; a gloss of coating film of usually 170 to 300%; a surface roughness Ra of coating film of usually not more than 11.5 nm; a Young""s modulus of usually 126 to 160; a surface resistivity value of usually not more than 1.0xc3x971010 xcexa9/cm2; a running durability of usually not less than 22 minutes; and a scratch resistance of usually A or B, when evaluated into four ranks: A (No scratch), B (A few scratches), C (Many scratches) and D (A great many scratches).
When the magnetic recording medium is produced by using the non-magnetic composite particles in which no hydroxides and/or oxides of aluminum and/or silicon coat is formed on the surface of the non-magnetic particles as core particles, the coercive force value thereof is usually 500 to 4,000 Oe (39.8 to 318.3 kA/m), preferably 550 to 4,000 Oe (43.8 to 318.3 kA/m); the squareness (residual magnetic flux density Br/saturation magnetic flux density Bm) thereof is usually 0.85 to 0.95, preferably 0.86 to 0.95; the gloss of coating film thereof is usually 170 to 300%, preferably 175 to 300%; the surface roughness Ra of coating film thereof is usually not more than 11.5 nm, preferably 2.0 to 11.0 nm, more preferably 2.0 to 10.5 nm; the Young""s modulus thereof is usually 126 to 160, preferably 128 to 160; the surface resistivity value thereof is usually not more than 1.0xc3x971010 xcexa9/cm2, preferably not more than 9.0xc3x97109 xcexa9/cm2, more preferably not more than 8.0xc3x97109 xcexa9/cm2; the running durability thereof is usually not less than 22 minutes, preferably not less than 24 minutes; and the scratch resistance thereof is usually A or B, preferably A when evaluated by the four-rank evaluation method as described below.
When the magnetic recording medium is produced by using the non-magnetic composite particles in which the hydroxides and/or oxides of aluminum and/or silicon coat is formed on the surface of the non-magnetic particles as core particles, the coercive force value thereof is usually 500 to 4,000 Oe (39.8 to 318.3 kA/m), preferably 550 to 4,000 Oe (43.8 to 318.3 kA/m); the squareness (residual magnetic flux density Br/saturation magnetic flux density Bm) thereof is usually 0.85 to 0.95, preferably 0.86 to 0.95; the gloss of coating film thereof is usually 175 to 300%, preferably 180 to 300%; the surface roughness Ra of coating film thereof is usually not more than 11.0 nm, preferably 2.0 to 10.5 nm, more preferably 2.0 to 10.0 nm; the Young""s modulus thereof is usually 128 to 160, preferably 130 to 160; the surface resistivity value thereof is usually not more than 1.0xc3x971010 xcexa9/cm2, preferably not more than 9.0xc3x97109 xcexa9/cm2, more preferably not more than 8.0xc3x97109 xcexa9/cm2; the running durability thereof is usually not less than 23 minutes, preferably not less than 25 minutes; and the scratch resistance thereof is usually A or B, preferably A when evaluated by the four-rank evaluation method as described below.
Under the consideration of high-density recording, the magnetic recording medium produced by using the magnetic metal particles containing iron as a main component or the acicular magnetic iron alloy particles as the magnetic particles, and the non-magnetic composite particles in which no hydroxides and/or oxides of aluminum and/or silicon coat is formed on the surface of the non-magnetic particles as core particles, has a coercive force value of usually 800 to 3,500 Oe (63.7 to 278.5 kA/m), preferably 900 to 3,500 Oe (71.6 to 278.5 kA/m), a squareness (residual magnetic flux density Br/saturation magnetic flux density Bm) of usually 0.87 to 0.95, preferably 0.88 to 0.95; a gloss of coating film of usually 195 to 300%, preferably 200 to 300%; a surface roughness Ra of coating film of usually not more than 9.0 nm, preferably 2.0 to 8.5 nm, more preferably 2.0 to 8.0 nm; a Young""s modulus of usually 128 to 160, preferably 130 to 160; a surface resistivity value of usually not more than 1.0xc3x971010 xcexa9/cm2, preferably not more than 9.0xc3x97109 xcexa9/cm2, more preferably not more than 8.0xc3x97109 xcexa9/cm2; a running durability of usually not less than 24 minutes, preferably not less than 26 minutes; and a scratch resistance of B or A, preferably A when evaluated by the four-rank evaluation method as described below.
The magnetic recording medium produced by using the magnetic metal particles containing iron as a main component or the acicular magnetic iron alloy particles as the magnetic particles, and the non-magnetic composite particles in which the hydroxides and/or oxides of aluminum and/or silicon coat is formed on the surface of the non-magnetic particles as core particles, has a coercive force value of usually 800 to 3,500 Oe (63.7 to 278.5 kA/m), preferably 900 to 3,500 Oe (71.6 to 278.5 kA/m), a squareness (residual magnetic flux density Br/saturation magnetic flux density Bm) of usually 0.87 to 0.95, preferably 0.88 to 0.95; a gloss of coating film of usually 200 to 300%, preferably 205 to 300%; a surface roughness Ra of coating film of usually not more than 8.5 nm, preferably 2.0 to 8.0 nm, more preferably 2.0 to 7.5 nm; a Young""s modulus of usually 130 to 160, preferably 132 to 160; a surface resistivity value of usually not more than 1.0xc3x971010 xcexa9/cm2, preferably not more than 9.0xc3x97109 xcexa9/cm2, more preferably not more than 8.0xc3x97109 xcexa9/cm2; a running durability of usually not less than 25 minutes, preferably not less than 27 minutes; and a scratch resistance of B or A, preferably A when evaluated by the four-rank evaluation method as described below.
Next, the process for producing the non-magnetic composite particles of the present invention will now be described.
The non-magnetic composite particles according to the present invention can be produced by adhering the inorganic fine particles onto the surface of each non-magnetic particle as a core particle, adding tetraalkoxysilane to the non-magnetic particles on which the inorganic fine particles are adhered, and then heat-treating the resultant mixture.
The inorganic fine particles may be adhered onto the surface of each non-magnetic particle as a core particle by the following method. That is, the non-magnetic particles may be mechanically mixed and stirred with the inorganic fine particles composed of at least one inorganic compound selected from the group consisting of oxides, nitrides, carbides and sulfides of aluminum, zirconium, cerium, titanium, silicon, boron or molybdenum, or with an aqueous or alcoholic colloid solution containing the inorganic fine particles, and then the resultant mixture is dried. In the consideration of uniform adhesion of the inorganic fine particles onto the surface of each non-magnetic particle as a core particle, the mixing and stirring with the colloid solution containing the inorganic fine particles are preferred.
As the inorganic fine particles, there may be used either synthesized products or commercially available products.
As the colloid solution containing the inorganic fine particles, there may be exemplified a colloid solution containing fine particles composed of at least one inorganic compound selected from the group consisting of oxides, nitrides, carbides and sulfides of aluminum, zirconium, cerium, titanium, silicon, boron or molybdenum. For example, there may be exemplified a colloid solution containing aluminum oxide, zirconium oxide, cerium dioxide, titanium dioxide, silicon dioxide, aluminum nitride, silicon carbide and molybdenum disulfide.
As the colloid solution containing aluminum oxide fine particles, there may be used an alumina sol (produced by Nissan Kagaku Kogyo Co., Ltd.) or the like.
As the colloid solution containing zirconium oxide fine particles, there may be used NZS-20A, NZS-30A or NZS-30B (tradenames all produced by Nissan Kagaku Kogyo Co., Ltd.) or the like.
As the colloid solution containing cerium oxide fine particles, there may be used a ceria sol (produced by Nissan Kagaku Kogyo Co., Ltd.) or the like.
As the colloid solution containing titanium oxide fine particles, there may be used STS-01 or STS-02 (tradenames both produced by Ishihara Sangyo Co., Ltd.) or the like.
As the colloid solution containing silicon oxide fine particles, there may be used SNOWTEX-XS, SNOWTEX-S, SNOWTEX-UP, SNOWTEX-20, SNOWTEX-30, SNOWTEX-40, SNOWTEX-C, SNOWTEX-N, SNOWTEX-O, SNOWTEX-SS, SNOWTEX-20L or SNOWTEX-OL (tradenames, all produced by Nissan Kagaku Kogyo, Co., Ltd.) or the like.
The amount of the inorganic fine particles which are mechanically mixed and stirred therewith or the inorganic fine particles contained in the colloid solution is preferably 0.1 to 20 parts by weight (calculated as oxide, nitride, carbide or sulfide) based on 100 parts by weight of the non-magnetic particles as core particles. When the amount of the inorganic fine particles is less than 0.1 part by weight, the amount of the inorganic fine particles adhered onto the surface of each non-magnetic particle may be insufficient, so that the obtained non-magnetic composite particles may not show a sufficient polishing effect. When the amount of the inorganic fine particles is more than 20 parts by weight, the obtained non-magnetic composite particles exhibit a sufficient polishing effect. However, since the amount of the inorganic fine particles adhered onto the surface of each non-magnetic particle is too large, the inorganic fine particles tend to be desorbed or fallen-off from the surfaces of the non-magnetic particles, thereby failing to produce a magnetic recording medium having excellent durability and magnetic head cleaning property.
In order to uniformly adhere the inorganic fine particles onto the surface of each non-magnetic particle as a core particle, it is preferred that aggregated non-magnetic particles be previously deaggregated using a pulverizer.
As apparatus (a) for mixing and stirring the core particles with the inorganic fine particles to adhere onto the surface of each non-magnetic particles as core particles, and (b) for mixing and stirring tetraalkoxysilane with the particles whose the inorganic fine particles are adhered on the respective surfaces, there may be preferably used those apparatus capable of applying a shear force to the particles, more preferably those apparatuses capable of conducting the application of shear force, spaturate force and compressed force at the same time.
As such apparatuses, there may be exemplified wheel-type kneaders, ball-type kneaders, blade-type kneaders, roll-type kneaders or the like. Among them, wheel-type kneaders are preferred.
Specific examples of the wheel-type kneaders may include an edge runner (equal to a mix muller, a Simpson mill or a sand mill), a multi-mull, a Stotz mill, a wet pan mill, a Conner mill, a ring muller, or the like. Among them, an edge runner, a multi-mull, a Stotz mill, a wet pan mill and a ring muller are preferred, and an edge runner is more preferred.
Specific examples of the ball-type kneaders may include a vibrating mill or the like. Specific examples of the blade-type kneaders may include a Henschel mixer, a planetary mixer, a Nawter mixer or the like. Specific examples of the roll-type kneaders may include an extruder or the like.
After adhering the inorganic fine particles onto the surface of each non-magnetic particle as a core particle, tetraalkoxysilane is mixed and stirred therewith, and the resultant mixture is then heat-treated so as to fix or anchor the inorganic fine particles onto the non-magnetic particles through a silicon compound derived (produced) from the tetraalkoxysilane.
The conditions of the above mixing or stirring treatment may be appropriately controlled such that the linear load is usually 2 to 200 Kg/cm (19.6 to 1960 N/cm), preferably 10 to 150 Kg/cm (98 to 1470 N/cm), more preferably 15 to 100 Kg/cm (147 to 960 N/cm); and the treating time is usually 5 to 120 minutes, preferably 10 to 90 minutes. It is preferred to appropriately adjust the stirring speed in the range of usually 2 to 2,000 rpm, preferably 5 to 1,000 rpm, more preferably 10 to 800 rpm.
The amount of tetraalkoxysilane adhered is preferably 0.05 to 70 parts by weight based on 100 parts by weight of the non-magnetic particles as core particles. When the amount of tetraalkoxysilane adhered is less than 0.05 part by weight, it may be difficult to fix or anchor the inorganic fine particles onto the surface of each non-magnetic particle in an amount sufficient to exhibit a good polishing effect and improve a durability. When the amount of tetraalkoxysilane coated is more than 70 parts by weight, it is possible to fix or anchor a sufficient amount of the inorganic fine particles onto the surface of each non-magnetic particle. However, since the fixing or anchoring effect is already saturated, the use of such a too large coating amount of tetraalkoxysilane is meaningless.
The temperature of the heat-treatment of tetraalkoxysilane is usually 40 to 200xc2x0 C., preferably 60 to 150xc2x0 C. The heat-treating time is preferably from 10 minutes to 36 hours, more preferably from 30 minutes to 24 hours. Thus, when being heat-treated under the above conditions, tetraalkoxysilane is converted into a suitable silicon compound.
The non-magnetic particles as core particles may be previously coated with an undercoating material composed of at least one compound selected from the group consisting of a hydroxide of aluminum, an oxide of aluminum, a hydroxide of silicon and an oxide of silicon prior to adhering the inorganic fine particles thereonto.
The formation of the undercoat on the surface of the non-magnetic particles may be conducted by adding an aluminum compound, a silicon compound or both aluminum and silicon compounds capable of forming the undercoat, to a water suspension prepared by dispersing the non-magnetic particles in water, mixing and stirring the resultant mixture, and further properly adjusting the pH value of the obtained mixture, if required, thereby coating the surface of each non-magnetic particle with at least one compound selected from the group consisting of a hydroxide of aluminum, an oxide of aluminum, a hydroxide of silicon and an oxide of silicon. The thus obtained mixture is filtered, washed with water, dried and then pulverized. If required, the obtained particles may be further subjected to deaeration, compaction or other treatments.
As the aluminum compounds used for forming the undercoat, there may be exemplified aluminum salts such as aluminum acetate, aluminum sulfate, aluminum chloride and aluminum nitrate; alkali aluminates such as sodium aluminate; or the like.
The amount of the aluminum compound added is usually 0.01 to 20% by weight (calculated as Al) based on the weight of the non-magnetic particles as core particles. When the amount of the aluminum compound added is less than 0.01% by weight, it may be difficult to obtain the effect of improving the desorption percentage of inorganic fine particles. When the amount of the aluminum compound added is more than 20% by weight, the effect of improving the desorption percentage of inorganic fine particles can be obtained. However, since the desorption percentage of inorganic fine particles-reduced effect is already saturated, it is meaningless to coat the non-magnetic particles with such a too large amount of the aluminum compound.
As the silicon compound used for forming the undercoat, there may be exemplified water glass #3, sodium orthosilicate, sodium metasilicate or the like.
The amount of the silicon compound added is usually 0.01 to 20% by weight (calculated as SiO2) based on the weight of the magnetic particles as core particles. When the amount of the silicon compound added is less than 0.01% by weight, it may be difficult to obtain the effect of improving the desorption percentage of inorganic fine particles. When the amount of the silicon compound added is more than 20% by weight, the effect of improving the desorption percentage of inorganic fine particles can be obtained. However, since the desorption percentage of inorganic fine particles-reduced effect is already saturated, it is meaningless to coat the non-magnetic particles with such a too large amount of the silicon compound.
In the case where the aluminum and silicon compounds are used in combination, the total amount of the aluminum and silicon compounds coated is usually 0.01 to 20% by weight (calculated as a sum of Al and SiO2) based on the weight of the non-magnetic particles as core particles.
Next, the process for producing the magnetic recording medium according to the present invention will be described.
The magnetic recording medium according to the present invention can be produced by applying a non-magnetic coating composition comprising the non-magnetic composite particles of the present invention, binder resin and solvent onto a non-magnetic base film to form a coating film, and drying the coating film to form a non-magnetic undercoat layer; and then by applying a magnetic coating composition comprising the magnetic particles, binder resin and solvent onto a non-magnetic undercoat layer to form a coating film, and then drying the coating film to form a magnetic recording layer.
As the solvent for the non-magnetic undercoat layer and magnetic recording layer, there may be used those generally used for the production of ordinary magnetic recording media. Examples of the solvents may include methyl ethyl ketone, toluene, cyclohexanone, methyl isobutyl ketone, tetrahydrofuran or mixtures thereof.
The amount of the solvent or solvents used is 65 to 1,000 parts by weight in total based on 100 parts by weight of the non-magnetic particles or magnetic particles. When the amount of the solvent used is less than 65 parts by weight, the obtained non-magnetic or magnetic coating composition may exhibit a too high viscosity, resulting in poor coatability thereof. When the amount of the solvent used is more than 1,000 parts by weight, a too large amount of the solvent may be volatilized upon coating which is disadvantageous from industrial viewpoints.
The important point of the present invention is that when non-magnetic composite particles obtained by adhering inorganic fine particles of at least one inorganic compound selected from the group consisting of oxides, nitrides, carbides and sulfides of aluminum, zirconium, cerium, titanium, silicon, boron or molybdenum, onto the surface of each non-magnetic particle as a core particle, and firmly fixing or anchoring the inorganic fine particles thereonto through a silicon compound derived (produced) from tetraalkoxysilane are used for the production of a non-magnetic undercoat layer of a magnetic recording medium, the obtained magnetic recording medium exhibits an excellent durability and a sufficient surface smoothness.
The reason why the non-magnetic undercoat layer having an excellent durability can be obtained by using the non-magnetic composite particles according to the present invention as non-magnetic particles therefor, is considered as follows. That is, since the inorganic fine particles having a high Mohs hardness such as oxide fine particles, nitride fine particles, carbide fine particles or sulfide fine particles used as a solid lubricant, are adhered on the surfaces of the non-magnetic particles, and further since these inorganic fine particles are fixed or anchored thereon through the silicon compound derived from tetraalkoxysilane so as to effectively prevent the inorganic fine particles from being desorbed or fallen-off from the surface of each non-magnetic particle, a sufficient polishing effect can be imparted to the non-magnetic particles.
Meanwhile, the reason why the inorganic fine particles are firmly fixed or anchored onto the surface of each non-magnetic particle, is considered as follows. That is, it is known that tetraalkoxysilane is readily hydrolyzed in the presence of water to produce silicon dioxide. In the present invention, the tetraalkoxysilane adhered is hydrolyzed by the interaction with a hydroxyl group derived from water absorbed on the surface of each non-magnetic particle and a hydroxyl group derived from water absorbed on the surfaces of the inorganic fine particles adhered onto the surface of each non-magnetic particle. Further, when the obtained particles are subjected to heat-dehydration, the inorganic fine particles are firmly fixed or anchored on the surface of each non-magnetic particle by the anchoring effect of the silicon compound derived from tetraalkoxysilane.
The reason why the non-magnetic composite particles according to the present invention can exhibit an excellent dispersibility, is considered as follows. That is, since fine irregularities are formed on the surfaces of the non-magnetic particles by adhering the inorganic fine particles thereonto, the contact area between the non-magnetic particles is reduced, and the particles are prevented from being agglomerated together, resulting in improved dispersibility thereof.
The magnetic recording medium provided with the non-magnetic undercoat layer using the non-magnetic composite particles according to the present invention have an excellent durability and a sufficient surface smoothness. The reason why the magnetic recording medium of the present invention can exhibit an excellent surface smoothness is considered as follows. That is, since the obtained non-magnetic undercoat layer has an excellent durability, the content of alumina having a deteriorated dispersibility can be reduced. Further, since the non-magnetic composite particles themselves have an improved dispersibility, the non-magnetic undercoat layer can be enhanced in surface smoothness.
When the non-magnetic composite particles according to the present invention are used for a non-magnetic undercoat layer of a magnetic recording medium, the obtained magnetic recording medium can show excellent durability and surface smoothness. Therefore, the non-magnetic composite particles according to the present invention are suitable as a material for high-density magnetic recording media.
Thus, the magnetic recording medium of the present invention obtained by using the above non-magnetic composite particles as non-magnetic particles for a non-magnetic undercoat layer thereof, can exhibit excellent surface smoothness and durability and, therefore, is suitable as a high-density magnetic recording medium.